Additive processing apparatus and method

ABSTRACT

An additive processing apparatus includes a chamber and an irradiation device that together are operable to additively fabricate an article from a powder layer-by-layer in a work space in the chamber. A gas recirculation loop is connected at a first end thereof to an outlet of the chamber and at a second end thereof to an inlet of the chamber. The gas recirculation loop includes at least one purification device that is configured to remove impurities from a cover gas and generate a clean cover gas.

BACKGROUND

Additive manufacturing typically involves building an article,layer-by-layer, from a powder material by consolidating selectedportions of each successive layer of powder until the complete articleis formed. Non-consolidated portions of the powder remain as a powderand are later removed. The additive manufacturing process can beconducted in a chamber with an inert gas, under controlled pressure andtemperature conditions. A laser beam can be used to melt and consolidatethe powder. For aerospace articles, such as airfoils, the powder istypically a metallic alloy powder.

SUMMARY

An additive processing apparatus according to an example of the presentdisclosure includes a chamber and an irradiation device togetheroperable to additively fabricate an article from a powder layer-by-layerin a work space in the chamber. A gas recirculation loop is connected ata first end thereof to an outlet of the chamber and at a second endthereof to an inlet of the chamber. The gas recirculation loop includesat least one purification device configured to remove impurities from acover gas and to generate a clean cover gas.

In a further embodiment of any of the present disclosure, the at leastone purification device includes a gas scrubber.

In a further embodiment of any of the present disclosure, the at leastone purification device includes a particle filter.

In a further embodiment of any of the present disclosure, the particlefilter is upstream of the gas scrubber with respect to flow from thefirst end to the second end.

A further embodiment of any of the present disclosure includes anadditional particle filter downstream of the gas scrubber with respectto flow from the first end to the second end.

In a further embodiment of any of the present disclosure, the gasscrubber includes a bed of granules.

In a further embodiment of any of the present disclosure, the gasscrubber includes at least one of activated carbon, magnesium, titanium,and copper.

In a further embodiment of any of the present disclosure, the gasscrubber includes a molecular sieve.

In a further embodiment of any of the present disclosure, the gasrecirculation loop splits into first and second passages including,respectively, first and second purification devices, and the first andsecond passages combine in the gas recirculation loop downstream of thefirst and second purification devices with respect to flow from thefirst end to the second end.

In a further embodiment of any of the present disclosure, the gasrecirculation loop includes at least one sensor downstream of the atleast one purification device with respect to flow from the first end tothe second end, the at least one sensor configured to detect acharacteristic representative of impurity level of the clean cover gas.

In a further embodiment of any of the present disclosure, the gasrecirculation loop includes a heat exchanger downstream of the at leastone purification device with respect to flow from the first end to thesecond end.

A further embodiment of any of the present disclosure includes a covergas source connected to the chamber for providing new cover gas, and acontroller configured to meter an amount of new cover gas with respectto an amount of clean cover gas circulated into the chamber from the gasrecirculation loop.

An additive processing method according to an example of the presentdisclosure includes providing a cover gas in a chamber in connectionwith additive fabrication of an article in the chamber, the cover gasentraining impurities during the additive fabrication, circulating thecover gas with entrained impurities from the chamber into a gasrecirculation loop, removing the entrained impurities from the cover gasin the gas recirculation loop to generate a clean cover gas, andcirculating the clean cover gas into the chamber during the additivefabrication.

A further embodiment of any of the present disclosure includes removinggaseous impurities using a gas scrubber.

A further embodiment of any of the present disclosure includes removingsolid impurities using a particle filter.

A further embodiment of any of the present disclosure includes removingthe solid impurities from the cover gas prior to removing the gaseousimpurities from the cover gas.

A further embodiment of any of the present disclosure includes coolingthe clean cover gas prior to circulating the clean cover gas into thechamber.

In a further embodiment of any of the present disclosure, thecirculating of the clean cover gas into the chamber includes introducingthe clean cover gas at a powder bed in the chamber.

An additive processing apparatus according to an example of the presentdisclosure includes a chamber and an irradiation device that aretogether operable to additively fabricate an article from a powderlayer-by-layer in a work space in the chamber. A gas recirculation loopis connected at a first end thereof to an outlet of the chamber and at asecond end thereof to an inlet of the chamber, to purify a cover gasreceived from the chamber and generate a clean cover gas circulated intothe chamber. The gas recirculation loop includes, in flow serial orderfrom the first end to the second end, a particle filter configured toremove solid impurities from the cover gas, a gas scrubber configured toremove gaseous impurities from the cover gas, and a heat exchangeroperable to cool the cover gas.

A further embodiment of any of the present disclosure, a cover gassource is connected to the chamber for providing new cover gas, with acontroller configured to meter an amount of new cover gas with respectto an amount of clean cover gas circulated into the chamber from the gasrecirculation loop.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example additive processing apparatus thatincludes a gas recirculation loop with at least one purification devicefor removing impurities from a cover gas to generate a clean cover gas.

FIG. 2 illustrates a further example that includes a particle filter anda gas scrubber.

FIG. 3 illustrates another example that includes a heat exchanger.

FIG. 4 shows another example that includes a cover gas source and acontroller for metering an amount of new cover gas provided into achamber.

FIG. 5 shows another example that includes at least one sensor in thegas recirculation loop.

FIG. 6 illustrates a further example in which a gas recirculation loopincludes at least one split for using several purification devices.

FIG. 7 illustrates a further example additive processing apparatus thatincludes a gas recirculation loop.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an additive processing apparatus 20.The additive processing apparatus 20 can be used for additivemanufacturing or fabrication techniques that involve building an articlelayer-by-layer from a powder material by consolidating selected portionsof each successive layer of powder until the complete article is formed.Additive manufacturing process can include, but are not limited to,selective laser melting, direct metal laser sintering, 3D printing,laser engineered netshaping (“LENS”) or laser powder forming. Such aprocess can be conducted in a chamber 22 with an inert cover gas, suchas argon, helium, or a mixture thereof (Ar/He), under controlledpressure and temperature conditions.

Some powders, such as metallic powders, are sensitive to moisture oroxygen in the processing environment, which can cause oxidation duringthe additive manufacturing process. Additionally, the powder itself maycontain minor amounts of impurities, such as moisture, oxygen, nitrogen,or other impurities. These impurities can be released into the inertcover gas during the additive manufacturing process. Additionally, theimpurities may build-up and foul components in the equipment, such asoptical lens components, used for transmitting a concentrated energybeam. Impurities can also contaminate sensors, cameras, or othercomponents in the chamber 22. To reduce build-up of impurities in theinert cover gas during the process, the gas can be vented and replacedwith new inert cover gas. This uses a relatively high amount of thecover gas, especially since additive fabrication can take a long periodof time in comparison to processes like welding. In this regard, theadditive processing apparatus 20 includes a gas recirculation loop 24that permits recycling of used cover gas from the chamber 22 by cleaningthe cover gas then returning it into the chamber 22. The removal of theimpurities can reduce the expenses related to the amount of cover gasused and contamination of components in the chamber 22.

In general, the additive processing apparatus 20 includes the chamber 22and an irradiation device 26 that together are operable to additivelyfabricate an article from a powder layer-by-layer in a work space,represented at W, in the chamber 22. Although not limited, theirradiation device 26 can be a laser or the like, that is capable ofdirecting a concentrated energy beam toward the work space to melt andconsolidate the selected powder that will be used to fabricate thearticle. For example, the powder can be, but is not limited to, ametallic alloy powder. With respect to the cover gas that is used duringthe additive manufacturing process within the chamber 22, the chamber 22includes a chamber outlet 22 a and a chamber inlet 22 b. One end of thegas recirculation loop 24 is connected to the outlet 22 a of the chamber22 and the other end of the gas recirculation loop 24 is connected tothe inlet 22 b of the chamber 22.

The gas recirculation loop 24 includes at least one purification device28 that is configured to remove impurities from the cover gas togenerate a clean cover gas. That is, during an additive manufacturingprocess within the chamber 22, the cover gas entrains impurities thatare then carried in the cover gas through the outlet 22 a into the gasrecirculation loop 24. The one or more purification devices 28 in thegas recirculation loop 24 serve to condition the used cover gas, therebycleaning the gas and providing the clean cover gas to be reintroducedinto the chamber 22 for the additive manufacturing process. In thisregard, the gas recirculation loop 24 can include a blower 30 forfacilitating movement of the cover gas through the gas recirculationloop 24. In one further example, the blower 30 is a variable speedcompressor to control or overcome pressure drop through the gasrecirculation loop 24. Additionally, if pressure drop is a concern,larger diameter piping, shallow adsorbent bed or beds, or both can beused. The purification device or devices 28 can be selected to removesolid impurities, gas impurities, moisture, or combinations thereof.

In a further example, the clean cover gas is selectively introduced backinto the chamber 22 at the powder bed during powder layer application,after powder layer application, during powder consolidation, or duringany combination of these. If introduced at a location remote from thepowder bed, the clean gas may be more likely to pick up impurities thatpotentially reduce the effectiveness of the clean cover gas before itever reaches the powder bed. For example, the clean cover gas isprovided in a laminar flow stream over the powder bed.

One or more cover gas sources 32 are operable to provide one or morecover gases into the chamber 22, or alternatively into the gasrecirculation loop 24. A valve 36 is provided to control flow of thecover gas into the chamber 22. The chamber 22 includes a vent or valve,represented at V, for venting the chamber 22 to facilitate controllingpressure. One or more sensors may be used in the chamber and/or in thegas recirculation loop 24 to detect pressure and facilitate operation ofthe additive processing apparatus 20. If the cover gas is a mixture ofgases, sensors can also be included to monitor and control thecomposition mixture.

FIG. 2 illustrates a modified example of an additive processingapparatus 120. In this disclosure, like reference numerals designatelike elements where appropriate and reference numerals with the additionof one-hundred or multiples thereof designate modified elements that areunderstood to incorporate the same features and benefits of thecorresponding elements. In this example, the gas recirculation loop 124includes at least two purification devices 128 a/128 b. In this example,the purification device 128 a is a particle filter and the purificationdevice 128 b is a gas scrubber. The particle filter is configured toremove solid impurities from the cover gas from the chamber 22 and thegas scrubber is configured to remove gaseous impurities from the covergas received from the chamber 22.

The particle filter is located upstream of the gas scrubber with respectto flow of the cover gas through the gas recirculation loop 124 from theoutlet 22 a to the inlet 22 b. Thus, prior to the cover gas beingreceived into the gas scrubber 128 b, particles are removed from thecover gas to avoid or reduce fouling of the gas scrubber.

In a further example, the gas scrubber includes granules 129 of amaterial that can remove gaseous impurities from the cover gas. Forexample, the granules 129 can be a molecular sieve material, an activecarbon material, or the like. In a further example, the granules 129 canadditionally or alternatively include an adsorbent to remove the gaseousimpurities from the cover gas. In a further example, additionally oralternatively, the granules 129 can include a catalytic material that isreactive with respect to target gaseous impurities in the cover gas tothereby trap the target gaseous impurities and remove them from thecover gas.

Example sieve materials can include activated carbon and activatedalumina. Examples of activated alumina can include BASF F-200, SelexsorbCD & Selexsorb COS.

Example adsorbents and catalyst materials can include manganese,titanium, copper, and combinations thereof. The manganese, titanium, andcopper are reactive with oxygen or moisture to form solid oxides andthus remove oxygen and moisture from the cover gas. One example coppercatalyst is BASF R3-11G. Additional catalyst examples are shown in thetable below.

Catalyst Main Constituents Contaminants Removed BASF R0-20 (3) Pd onAl₂O₃ O₂, H₂, CO, acetylenes, dienes, olefins, VOC's BASF R0-25 (3) Pdon Al₂O₃ Same as R0-20/47 BASF R3-11 (3) CuO & Mg—Si Special O₂-getterfor Indicating O₂ Traps BASF R3-11G (3) CuO & Mg—Si Oxygen from gases &liquids (regenerable) BASF R3-12 (3) CuO₊ ZnO & Al₂O₃ Arsine, phosphine,H₂S, COS, RSH BASF R3-15 (3) CuO₊ ZnO & Al₂O₃ Oxygen from gases &liquids (regenerable) BASF R3-16 (3) CuO₊ ZnO & Al₂O₃ Oxygen & CO fromethylene (primarily for PE/PP units) Engelhard Cu- CuO on Al₂O₃ Oxygenfrom gases & liquids (regenerable). See note 4. 0226S Engelhard DEOXOVarious PMs Types Various gas purifications. Engelhard Ni-3288 NiO onsupport Oxygen from specialty gases Engelhard Q-5 CuO on Al₂O₃ Oxygenfrom gases & liquids (regenerable). RC1 GetterMax CuO₊ ZnO & Al₂O₃ O₂from gases & liquids (regenerable) 133T

For example, R0-20/47 is 0.47 wt % Pd on 2-4 mm beads. R0-25 issupported on highly calcined extrudates (3 mm, 4 mm or 6 mm) and canhave Pd loadings of 0.15%, 0.3% or 0.50%, for use if temperatures exceedabout 600° C., or low pressure drop is needed. The Engelhard DEOXOcatalysts include a range of supported precious metals catalysts usingPd, Pt, Rh and/or Ru on alumina supports.

In a further example, the granules 129 of the gas scrubber can be in abed, such as a packed bed that is contained within a column, housing, orother structure, to avoid or reduce the chance that the granules orportions thereof are released into the cover gas and foul downstreamcomponents. In one further example, the containment structure isinsulated and includes a heater that is operable to selectively heat thebed for regeneration. During regeneration, a reducing gas can be used toreduce oxidized scrubber species, such as copper oxide, and to carryaway moisture. The direction of gas flow through the gas scrubber duringregeneration can be opposite the direction of flow through the gasscrubber during use to remove impurities.

Additionally, the particle filter 128 a, the gas scrubber 128 b, orboth, can be removable and/or regenerated and reused. For example, thegas scrubber 128 b can be removed from the additive processing apparatus120, regenerated, and then returned into the additive processingapparatus 120 for further use.

FIG. 3 shows another modified example of an additive processingapparatus 220. The additive processing apparatus 220 is similar to theapparatus 120 except that the gas recirculation loop 224 also includes aheat exchanger 230. In this example, the heat exchanger 230 is locateddownstream from the gas scrubber 128 b with respect to flow through thegas recirculation loop 224. Thus, after removal of particle impuritiesand gaseous impurities from the cover gas, the heat exchanger can beutilized to cool the clean cover gas prior to reintroduction into thechamber 22. The temperature of the clean cover gas provided back intothe chamber 22 can be controlled to facilitate control over theprocessing temperature in the chamber 22 during an additivemanufacturing process. Alternatively or additionally, a heat exchangercould be provided upstream of the particle filter 128 a and/or betweenthe particle filter 128 a and the gas scrubber 128 b.

For example, if the cover gas temperature exceeds the use temperature ofthe particular particle filter 128 a or gas scrubber 128 b, one or moreheat exchangers can additionally or alternatively be included to coolthe cover gas prior to the particle filter 128 a or the gas scrubber 128b. For instance, adsorbents, such as a molecular sieve for moisturecapture, have higher sorption capacity near room temperature. However,impurity particulates can build-up on the heat exchanger surfaces. Toreduce or avoid such build-up, the heat exchangers can be orientedvertically so that any particulates will fall into a dropout pot. Otherscrubbers may function at higher temperatures to promote reactions, andthus not need cooling of the cover gas. For example, titanium reactswith nitrogen at elevated temperatures to form titanium nitride.

FIG. 4 illustrates another modified example of an additive processingapparatus 320 that is somewhat similar to the apparatus 220 but alsoincludes a cover gas source 332 and a controller 334. The cover gassource 332 is connected to the chamber 22 for providing new cover gasinto the chamber as needed. The new cover gas can be provided directlyinto the chamber 22, as shown, or introduced into the gas recirculationloop 224. In this regard, the connection between the cover gas source332 and the chamber 22 includes a metering valve 336 a. The controller334 is in communication with the metering valve 336 a and can thuscontrol or meter the amount of new cover gas provided into the chamber22 during an additive manufacturing process. The controller 334 is alsoin communication with the gas recirculation loop 224 to monitor flow,pressure, or the like for determining or regulating the amount of cleancover gas provided back into the chamber 22 by controlling blower 30 andmetering valve 336 b. Typically, if the blower 30 is used forregulation, the metering valve 336 b will not be needed.

During an additive manufacturing process, cover gas can be ventedthrough a valve, as shown at V, from the chamber 22 to facilitatecontrolling the pressure and internal conditions within the chamber 22.Thus, cover gas can be lost during the additive manufacturing process.In this regard, the controller 334 can also be in communication with thevent, V, to monitor and control how much cover gas is vented from thechamber 22. The controller 334 can then control the amount of the newcover gas (i.e., make-up gas) provided from the cover gas source 332with respect to the amount of clean cover gas circulated back into thechamber 22 from the gas recirculation loop 224 to maintain pressure inthe chamber 22 to be within a target range. The controller 334 caninclude software, hardware, or a combination thereof that is programmedfor such functions.

FIG. 5 illustrates a modified example of another additive processingapparatus 420. In this example, the apparatus 420 is somewhat similar tothe apparatus 320 but also includes at least one sensor 438 in the gasrecirculation loop 424. The sensor or sensors 438 are configured todetect a characteristic that is representative of the impurity level ofthe clean cover gas in the gas recirculation loop 424. In this regard,at least one of the sensors 438 is located downstream from at least onepurification device. In this example, the sensor 438 is locateddownstream from the particle filter 128 a and the gas scrubber 128 b.For example, the sensor is configured to detect oxygen level, moisturelevel, or other gas level indicator which can be used to indicate theimpurity level of the clean cover gas. For instance, if there is amalfunction in one or more of the purification devices or thepurification device reaches its capacity limit, the sensor can detect arising impurity level and thus alert a user. The impurity level may alsoor alternatively be used to control the circulation rate of the covergas through the gas recirculation loop 424.

FIG. 6 shows another modified example of an additive manufacturingapparatus 520 that again is somewhat similar to the apparatus 420. Inthis example, the gas recirculation loop 524 includes one or more splitsthat enable the use of several purification devices. For example, thecover gas can be directed to one of the purification devices while theother is being changed or regenerated, and then subsequently directed tothe other purification device while the first purification device isbeing changed or regenerated. The changing or regeneration can bemanual, automatic, or semi-automatic. Thus, in this example, the gasrecirculation loop 524 includes two particle filters 528 a ₁ and 528 a ₂that are arranged in the split. A valve 540 may be provided at the splitto control which of the particle filters 528 a ₁ or 528 a ₂ the covergas flows to.

Additionally or alternatively, a split can be provided for several gasscrubbers. As an example, the gas recirculation loop 524 also includes asplit for a first gas scrubber 528 b ₁ and a second gas scrubber 528 b₂. Similarly, a valve 542 is provided upstream of the gas scrubbers 528b ₁ and 528 b ₂ for controlling which of the gas scrubbers the cover gasflows to. Similar to the particle filters, the cover gas can be directedto one of the gas scrubbers while the other is being changed orregenerated, and then subsequently directed to the other gas scrubberswhile the first is being changed or regenerated. The changing orregeneration can be manual, automatic, or semi-automatic. Additionally,the valves 540 and 542 can be connected to the controller 334 forcontrolling the operation thereof. Alternatively, the valves 540 and 542can be manually operable. Each split combines back into the gasrecirculation loop 524 downstream of the purification devices.

In this example, the gas recirculation loop 524 also includes one ormore additional purification devices 28. For example, one or moreadditional purification devices 28 can be provided downstream of the gasscrubber or scrubbers to remove any particles that may become entrainedin the cover gas from the granules in the gas scrubber. The additionalpurification device, such as a particle filter or HEPA filter, can beprovided prior to the heat exchanger 230 and, optionally, anotherpurification device, such as another particle filter or HEPA filter, canbe provided downstream of the heat exchanger 230.

In one further modification, there will be an initial bulk particlefilter (e.g., 128 a in FIG. 5) for removing the largest particles,followed by a fine particle filter and/or HEPA filter for removingsmaller or submicron particles. The bulk particle filter can include adropout pot having a container with an inlet pipe through which the gasand particulates enter. The large particles and gas travel downward andthe large particles drop out when the gas changes direction and travelsupward to an outlet. Such as device could alternatively also be acyclone. The gas with fine particles will then travel to the filterelement where the gas needs to pass through the filter and the particlesstick to the outside of the filter surface (and some penetrate). Thesesmall particles form a filter cake. That filter cake can be broken upand released from the filter by a pressure backpulse. The filter cakethen drops to the bottom where it can be collected.

A first gas scrubber follows the fine particle filter and/or HEPAfilter, to remove gas impurities that can foul a second downstream gasscrubber. For example, the second gas scrubber includes a catalyst ormolecular sieve that removes oxygen/moisture and the first gas scrubberincludes activated carbon that removes gases such as nitrogen, methane,and carbon oxides that can hinder the catalyst or molecular sieve fromremoving the oxygen/moisture. A hot getter at approximately 400-1000° C.can be provided downstream from the second gas scrubber to furtherremove remaining trace gas species, such as carbon oxides, methane,nitrogen, oxygen, and ammonia, which may otherwise influence theadditive fabrication.

The examples discussed above also represent an additive processingmethod. Such a method can include providing the cover gas to the chamber22 in connection with an additive fabrication of an article in thechamber 22. The cover gas entrains impurities during the additivefabrication and is then circulated with the entrained impurities fromthe chamber 22 into the gas recirculation loop 24/124/224/424/524. Theentrained impurities are then removed from the cover gas in the gasrecirculation loop to generate the clean cover gas. The clean cover gasis then circulated back into the chamber 22 during the additivefabrication. As in the examples, the removal of the impurities can beconducted using a gas scrubber, a particle filter, or combinationsthereof as discussed above. Additionally, the clean cover gas can thenbe cooled prior to circulating the clean cover gas back into the chamber22. It is to be understood that the examples above described withrespect to FIGS. 1-6 are hereby incorporated as further examples of themethod, in that any or each of the functions of the components of thedescribed examples can also represent a step or action in the method.

In one further example, the gas recirculation loop 24/124/224/424/524 isutilized after a purge process that controls the oxygen/moisture contentin the chamber 22 to be below a threshold which the gas recirculationloop 24/124/224/424/524 can handle. For example, the oxygen/moisturecontent is lowered to below 100 part-per-million by venting used covergas from the chamber 22 and providing new cover gas. Thus, the chambermay be used in an open-loop configuration until the level is reducedbelow the threshold, which then triggers activation of the gasrecirculation loop 24/124/224/424/524 in a closed-loop mode. Limitingthe exposure level of the gas recirculation loop 24/124/224/424/524 mayserve to reduce the frequency or need for replacing or regeneratingpurification devices.

In one further example, lowering the oxygen/moisture level can beaccelerated by providing a balloon or other expandable reservoir ofcover gas in the chamber 22. The cover gas in the chamber 22 outside ofthe balloon must flow around the balloon into corners or other low-flowareas, to facilitate purging those areas. The balloon is then deflatedand removed prior to the additive fabrication process. Depending on theparticular additive fabrication process and components within thechamber 22, there may be more or less space for such a balloon.

FIG. 7 shows another modified example of an additive manufacturingapparatus 620. In this example, the first particle filter 528 a ₁ (shownwith dropout pot, as described above) and the second particle filter 528a ₂ (shown with dropout pot, as described above) are arranged in seriesin the gas recirculation loop 624. The first particle filter 528 a ₁ isconfigured to remove relatively large particles, and the second particlefilter 528 a ₂ is configured to remove smaller particles, with blowbackpulse for continuous filtration. The gas scrubber 128 b in this exampleincludes a heater 631 and is arranged downstream from the second gasscrubber 528 b ₂. In addition to cover gas source 332, there is also areducing gas source 332 a. As shown, the gas recirculation loop 624includes a plurality of sensors, generally designated as 650, and aplurality of valves, generally designated as 652, for further monitoringand controlling the apparatus 620. One or more of the sensors is anoxygen sensor and one or more of the sensors is a water/moisture sensor.The sensor 650 from the chamber 22 is a pressure sensor. It is to beunderstood that any or all of the illustrated components can be incommunication with a controlling having software, hardware, or both forcontrolling the operation thereof.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

What is claimed is:
 1. An additive processing apparatus comprising: achamber and an irradiation device together operable to additivelyfabricate an article from a powder layer-by-layer in a work space in thechamber; a gas recirculation loop connected at a first end thereof to anoutlet of the chamber and at a second end thereof to an inlet of thechamber, the gas recirculation loop including at least one purificationdevice configured to remove impurities from a cover gas and to generatea clean cover gas, wherein the gas recirculation loop includes a heatexchanger downstream of the at least one purification device withrespect to flow from the first end to the second end a cover gas sourceconnected to the chamber for providing new cover gas; and a controllerconfigured to meter an amount of new cover gas with respect to an amountof clean cover gas circulated into the chamber from the gasrecirculation loop.
 2. The apparatus as recited in claim 1, wherein theat least one purification device includes a gas scrubber.
 3. Theapparatus as recited in claim 2, wherein the at least one purificationdevice includes a particle filter.
 4. The apparatus as recited in claim3, wherein the particle filter is upstream of the gas scrubber withrespect to flow from the first end to the second end.
 5. The apparatusas recited in claim 4, further comprising an additional particle filterdownstream of the gas scrubber with respect to flow from the first endto the second end.
 6. The apparatus as recited in claim 2, wherein thegas scrubber includes a bed of granules.
 7. The apparatus as recited inclaim 2, wherein the gas scrubber includes at least one of activatedcarbon, magnesium, titanium, and copper.
 8. The apparatus as recited inclaim 2, wherein the gas scrubber includes a molecular sieve.
 9. Theapparatus as recited in claim 1, wherein the gas recirculation loopsplits into first and second passages including, respectively, first andsecond purification devices, and the first and second passages combinein the gas recirculation loop downstream of the first and secondpurification devices with respect to flow from the first end to thesecond end.
 10. The apparatus as recited in claim 1, wherein the gasrecirculation loop includes at least one sensor downstream of the atleast one purification device with respect to flow from the first end tothe second end, the at least one sensor configured to detect acharacteristic representative of impurity level of the clean cover gas.11. An additive processing apparatus comprising: a chamber and anirradiation device together operable to additively fabricate an articlefrom a powder layer-by-layer in a work space in the chamber, the chamberincluding a vent valve; a gas recirculation loop connected at a firstend thereof to an outlet of the chamber and at a second end thereof toan inlet of the chamber and, to purify a cover gas received from thechamber and generate a clean cover gas circulated into the chamber, thegas recirculation loop including, in flow serial order from the firstend to the second end, a particle filter configured to remove solidimpurities from the cover gas, a gas scrubber configured to removegaseous impurities from the cover gas, a heat exchanger operable to coolthe cover gas, and a recirculation loop metering valve; a cover gassource and a cover gas metering valve, the cover gas source beingconnected through the cover gas metering valve to the chamber forproviding new cover gas; and a controller connected with the vent valve,the recirculation loop metering valve, and the cover gas metering valve,the controller configured to operate the vent valve to vent the covergas from the chamber through the vent valve and operate the cover gasmetering valve and the recirculation loop metering valve to meter anamount of new cover gas provided into the chamber as make-up gas for thecover gas that is vented from the vent valve with respect to an amountof clean cover gas circulated into the chamber from the gasrecirculation loop.
 12. The apparatus as recited in claim 1, wherein theat least one purification device includes, in serial flow order, aparticle filter and a gas scrubber.
 13. The apparatus as recited inclaim 1, further comprising a metering valve in the gas recirculationloop and a controller in communication with the metering valve andconfigured to regulate an amount of clean cover gas from the gasrecirculation loop into the chamber.
 14. The apparatus as recited inclaim 11, wherein the particle filter is upstream of the gas scrubberwith respect to flow from the first end to the second end.
 15. Theapparatus as recited in claim 14, further comprising an additionalparticle filter downstream of the gas scrubber with respect to flow fromthe first end to the second end.
 16. The apparatus as recited in claim14, wherein the gas scrubber includes at least one of activated carbon,magnesium, titanium, and copper.
 17. The apparatus as recited in claim14, wherein the gas recirculation loop includes at least one sensorconfigured to detect a characteristic representative of impurity levelof the clean cover gas.
 18. An additive processing method forfabricating an article using the additive processing apparatus asrecited in claim 1, including circulating the cover gas with impuritiesfrom the chamber into the gas recirculation loop, removing theimpurities from the cover gas in the gas recirculation loop using thepurification device to generate the clean cover gas, circulating theclean cover gas into the chamber during the additive fabrication, andmetering the amount of new cover gas with respect to the amount of cleancover gas circulated into the chamber from the gas recirculation loopusing the controller.